API Moved "SideBar" widget relation to "widgets" module

Removed BlogTreeExtension in favour of the new WidgetPageExtension
in the "widgets module". Use the following code to update:

Object::add_extension('BlogTree', 'WidgetPageExtension');
Object::add_extension('BlogEntry', 'WidgetPageExtension');

The template placeholder has been renamed from $SideBar to
$SideBarView. In case you're overriding blog templates,
please adjust accordingly.

This change means that widgets can theoretically be applied
to individual blog entries as well, since they need the
"SideBar" relationship as well. In practice, this defaults
to "InheritSideBar" though, which pulls them from the BlogHolder as usual.

Relies on changes to widget, see https://github.com/silverstripe/silverstripe-widgets/commit/d83f0d2ae0ac0f96534ad752bc2b5531af8549b8

I've applied this to 0.6 instead of master in order to have the change compatible with SS 3.0, otherwise I feel that adoption of this stuff will be too slow if we make it 3.1 only